Numerous semiconductor devices and assemblies may be formed utilizing silicon-on-insulator (SOI) constructions. Such assemblies can include, for example, fully depleted SOI devices or partially-deleted SOI devices. Among SOI devices are n-channel transistors and p-channel transistors. Such transistors can, depending on the desired characteristics, be either fully depleted SOI devices or partially depleted SOI devices. Also, such transistors can be incorporated into specific types of devices, such as, for example, memory array transistor devices and peripheral transistor devices.
There are generally two ways of providing a starting substrate for SOI fabrication. In a first method, oxygen is implanted at a desired depth into a silicon wafer. The wafer is then subjected to an anneal to form a buried silicon dioxide layer having an outward monocrystalline silicon layer thereover. The anneal can also repair damage caused by the implant, although the repair is typically not perfect.
In a second method, a silicon wafer is initially provided with an outer silicon dioxide layer. Such outer silicon dioxide layer can be formed, for example, by exposing the wafer to an oxidizing ambient. After formation of the outer silicon dioxide layer, a separate silicon wafer is positioned against the silicon dioxide layer to form a composite comprising the silicon dioxide layer sandwiched between a pair of silicon wafers. The composite is heated in a furnace to cause fusing of the silicon wafers with the silicon dioxide. Thereafter, the second silicon wafer is mechanically polished down to a desired thickness such that its remnants constitute an SOI construction.
In many applications, it is desired to have an SOI construction in which the silicon layer has a substantially uniform thickness throughout its construction. A method for improving the uniformity of thickness of a silicon layer in an SOI construction is described with reference to FIG. 1, which illustrates an apparatus 10 configured for electrostatically etching a silicon layer of a SOI construction. Apparatus 10 comprises a vessel 12 within which is an etching composition 14 preferably comprising potassium hydroxide. A heater 16 is provided within etching composition 14 to control a temperature of the composition during an etching process. An SOI construction 18 is supported within vessel 12 by a TEFLON.TM. holder 20 which comprises a back support 22 and a front support 24. SOI construction 18 is compressed between back support 22 and front support 24. A first O-ring 26 is between SOI construction 18 and front support 24 and seals a back of SOI construction 18 from exposure to etching composition 14. A second O-ring 34 seals front support 24 against back support 22. An electrode 28 extends across a back of SOI construction 18 and supports the back of SOI construction 18 while also providing an electrical connection to SOI construction 18. Electrode 28 is electrically connected through a voltage supply 30 to a platinum electrode 32 extending within etching composition 14.
An expanded view of zone 2 of FIG. 1 is shown in FIG. 2. As shown, SOI construction 18 comprises a substrate portion 40, an insulator layer 42, and a silicon layer 44. Silicon layer 44 is a "frontside" of construction 18 and substrate 40 is a "backside" of construction 18. Frontside 44 is exposed to etching composition 14.
In operation, silicon layer 44 is generally lightly doped, with, for example, a p-type conductivity-enhancing dopant. A voltage is provided with voltage supply 30 to force a depletion region 46 to be formed within silicon layer 44. Etching composition 14 then etches silicon layer 44 to about depletion region 46 and stops. A thickness of depletion region 46 can be controlled by controlling a voltage provided by voltage supply 30. Although it is not clear if the etching composition stops etching at the depletion layer, or at some location near the depletion layer, it is clear that the amount of silicon etched from layer 44 can be controlled by controlling a thickness of depletion layer 46.
In operation, a 20% (wt) potassium hydroxide solution is typically used as an etchant and 50-75 volts are applied by voltage supply 30 for a typical etching duration of about six minutes. The temperature of the potassium hydroxide solution is typically controlled to be about 70.degree. C. with temperature controller 16.
The above-discussed methods of forming SOI constructions are utilized to form constructions in which the silicon layer has a relatively uniform thickness. However, in accordance with the invention which follows it is recognized that there may be some applications in which it is desirable to form SOI constructions having a silicon layer of varying thickness. Accordingly, methods are described for creating SOI constructions in which the silicon layer has a varying thickness. Also described are assemblies and devices designed to take advantage of an SOI construction within which the silicon layer has a varied thickness.